qemu-io: Make 'open' subcommand more like command line

qemu-io: Make 'open' subcommand more like command line

The command line defaults to BDRV_O_UNMAP, but can use
-d to reset it.  Meanwhile, the 'open' subcommand was
defaulting to no discards, with no way to set it.

The command line has both -n and -tMODE to set a variety
of cache modes, but the 'open' subcommand had only -n.

The 'open' subcommand had no way to set BDRV_O_NATIVE_AIO.

Note that the 'reopen' subcommand uses '-c' where the
command line and 'open' use -t.  Making that consistent
would be a separate patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 1462677405-4752-3-git-send-email-eblake@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>